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General Astronomy Stars and Galaxies

by: Stephan Kuvalis

General Astronomy Stars and Galaxies ASTR 1120

Marketplace > University of Colorado at Boulder > Astronomy > ASTR 1120 > General Astronomy Stars and Galaxies
Stephan Kuvalis

GPA 3.89


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This 5 page Class Notes was uploaded by Stephan Kuvalis on Thursday October 29, 2015. The Class Notes belongs to ASTR 1120 at University of Colorado at Boulder taught by Staff in Fall. Since its upload, it has received 14 views. For similar materials see /class/231961/astr-1120-university-of-colorado-at-boulder in Astronomy at University of Colorado at Boulder.


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Date Created: 10/29/15
ASTR 1120 Section 1 3 credit hours Spring 2006 SUMMARY OF KEY CONCEPTS GALAXIES AND COSMOLOGY Lecture 17 textbook Chapter 20 We discussed galaxies a galaxy is an island of stars held together as a single unit by gravity A large galaxy such as the Milky Way galaxy that the Sun is part of contains around 100 billion stars There are about 100 billion galaxies in the observable Universe Galaxies are observed to often live in groups or clusters of galaxies There is no clear demarcation between these but a group normally refers to a system with a handful of large galaxies while a cluster is a much richer structure sometjlnes containing hundreds of large galaxies The Milky Way belongs to the Local Group which also includes Andromeda the nearest large galactic neighbor to the Milky Way An ilnportant distinction between stars and galaxies is that galaxies are separated by distances that are typically around 10 times the size of a galaxy This means that galaxies are packed together relatively tightly and as they orbit around under the action of gravity they often collide with each other Those collisions lead to mergers and allow large galaxies to grow by swallowing smaller ones This is unlike the situation for stars stars are extremely small compared to the typical separation between stars and as a result stars almost never collide Galaxies are classified based primarily on their appearance The main distinction is between elliptical galaxies and spiral galaxies the Milky Way is a spiral with the latter being further divided into barred and unbarred spirals Elliptical galaxies are smooth round and rather featureless They lack a lot of cool gas and as a result are not forming stars today and don t have spiral arms or dust lanes Spiral galaxies are defined by their spiral arms which can be prominent due to either recent star formation or dust lanes Irregular galaxies are another class these have a disturbed morphology that is often the result of ongoing interactions or mergers with other galaxies Lecture 18 Hubble s Law relates the distance of a galaxy from us to its recession velocity The law can be written as vH0d where H0 is called Hubble s constant This law is important make sure you know what it means Hubble s Law requires measurement of the distances and velocities of distant galaxies Distance is hard to measure because a faint galaxy could either be intrinsically faint and relatively nearby or very luminous and distant In other words measuring the apparent brightness tells us nothing about the distance recall the inverse square law However for some sources called standard candles the luminosity is known independently in the simplest case the luminosity of some class of objects is fixed In this case the apparent brightness does tell us the distance White dwarf supernovae triggered when a white dwarf star gains mass and exceeds the maximum mass a stable white dwarf can have are excellent standard candles and the best way to measure distances to distant galaxies Velocities can be measured from the shift in the wavelengths of spectral lines caused by the Doppler shift this is much easier than measuring distances If a spectral line observed on Earth to have a wavelength he is seen at wavelength Kobsewed in the spectrum of a distant galaxy we define the redshift 2 via 2 KobsewedAres he Distant galaxies have large redshifts For small redshifts the redshift can be related to the recession velocity via a simple formula v cz where c is the speed of light Lecture 19 We discussed Olber s Paradox why is the night sky dark In an infinite static everlasting Universe every line of sight would eventually intercept the surface of a star whose surface brightness is independent of the distance So the whole sky would be the same brightness as the disk of the Sun That this is not the case is the paradox The resolution of the paradox lies in the fact that the Universe is neither static nor everlasting The Big Bang is a theory of cosmology in which the Universe at early times was hot and dense Over time it has expanded and cooled We observe the expansion as Hubble s Law distant galaxies are moving away from us at a rate that increases the further away they are Hubble s Law does not imply that there is a single center to the Universe or that the Universe is expanding into something Observers in other galaxies would determine the same law as we do A good analogy for the expansion of the Universe is to think about what happens to dots painted on the surface of a balloon as it s blown up each dot gets further away from every other dot with time We showed that the age of the Universe can be estimated in a simple way once Hubble s constant is known in fact the age l H0 once H0 is expressed in appropriate units A high value of Hubble s constant means a young Universe and vice versa The observed value of Hubble s constant implies the Universe is about 136 billion years old Lecture 20 textbook Chapter 19 sections dealing with dark matter The luminous mass in a galaxy can be estimated by measuring the light from stars and adding in mass in the form of gas that can be detected with radio telescopes We can also estimate the total mass by studying the motion of stars and gas and applying Newton s Laws of motion in particular the formula that we used previously to measure the mass of the black hole at the Galactic Center In the context of a galaxy the formula allows us to determine the mass M of the galaxy within some radius r if we can measure the velocity V of stars or gas at that radius This analysis is easiest to apply to spiral galaxies which have a wellorganized rotation of the stars and gas about the center In most cases the resulting rotation curve velocity as a function of radius is found to be at at large radius This is an important observation It implies that there is more mass in spiral galaxies than can be accounted for by the observed stars and gas This is called dark matter matter which affects stars and gas via gravitational forces this is how we infer it exists but which does not emit much or any light or other observable radiation The dark matter appears to be distributed in a halo a roughly spherical structure around the galaxy that extends beyond the luminous edges defined by the spiral disk Dark matter could be many things baryonic dark matter of which there are many examples eg low mass stars or stellar remnants that is made of the same stuff neutrons and protons as ordinary stars or nonbaryonic dark matter made up of some elementary particle that has yet to be directly discovered By a process of elimination most astronomers believe the dark matter is made up of a new elementary particle and experiments to detect such a particle on Earth are underway Lecture 21 Chapter 22 Dark matter unseen mass whose existence is inferred from its gravitational influence on stars and gas Dark energy unseen substance that appears to be causing the rate of expansion of the Universe to be accelerating the opposite of what gravity is expected to do In addition to the evidence from spiral galaxy rotation curves additional evidence for the existence of dark matter comes from observations of galaxy clusters Clusters are the largest gravitationally bound structures in the Universe which means that they provide a fair sample of the cosmic inventory of ordinary matter and dark matter The amount of dark matter in a galaxy cluster can be estimated in three independent ways 1 Measuring the velocities of galaxies and applying the same sort of analysis as for spiral galaxies more mass whether luminous or dark means higher velocities for galaxies moving around within the cluster 2 Measuring the temperature of gas distributed smoothly in the cluster potential more mass means a higher temperature This gas is hot often 10 million degrees or more so it emits in the Xrays 3 Analysis of gravitational lensing all mass in the cluster combines to de ect the light of background galaxies as that light travels through the cluster The lensing distorts the images of background galaxies into rings arcs and multiple images The distortion can be measured and used to infer the mass The result is that on the scales of clusters and by extension for the Universe as a whole about 15 of the mass is in the form of ordinary matter stars and gas while the other 85 is dark matter Lecture 22 Chapter 23 We discussed the evidence for the hot Big Bang The basic idea is that Hubble s Law implies that the Universe was denser in the past When a gas expands it cools so the Universe would have been hotter as well as denser at early times At very early times the temperatures would have exceeded any that can be attained in experiments on Earth today so the early Universe acted like a huge particle accelerator As the Universe cools starting from very high temperature a critical moment arrives when the Universe is first cool enough for atoms to form This moment is called recombination Prior to recombination the Universe contains a plasma free electrons and protons which is opaque to photons After recombination the Universe contains atoms electrons bound to protons in hydrogen and heliuln atoms and is transparent to photons At recombination photons that were previously trapped within the plasma are released they then travel freely through the Universe without further interaction with matter and can be observed today as the Cosmic Microwave Background CMB The CMB is made up of microwaves because the expansion of the Universe redshifts the radiation from the original red infrared the type of radiation emitted when atoms form at T 3000K by a factor of more than a 1000 The CMB is observed to have properties consistent with those expected from the Big Bang theory 1 It s thermal radiation the type of radiation produced from opaque bodies The temperature is 273K the lowest naturally occurring temperature in the Universe 2 It comes from all directions on the sky not clustered like stars and galaxies are In more detail the CMB sky is not completely uniform 1 There is a large scale pattern caused by the motion of the Milky Way relative to the frame in which recombination occurred This motion a few hundred km per second means that the CMB is Doppler shifted it s a few thousandths of a degree hotter in the direction toward which our galaxy is moving and cooler in the opposite direction 2 If this is subtracted there s an even smaller pattern of ripples with hot and cool spots that are a few parts in 100000 different from the mean temperature These anisotropies are the seeds from which galaxies and clusters of galaxies formed viewed when the Universe was only 380000 years old Lecture 23 The Universe would have been hot enough for fusion reactions to occur within the first three minutes or so after the Big Bang This era is called nucleosynthesis Big Bang nucleosynthesis forms hydrogen heliuln and some lithium the other heavier elements are formed within stars We discussed the puzzle of why the Universe appears to be surprisingly uniform on very large scales as evidenced by the distribution of very distant galaxies on scales much larger than clusters of galaxies and especially by the uniformity of the CMB sky once the Galaxy s motion has been subtracted This is a puzzle because widely separated points on the sky have never been in contact with each other during the history of the Universe according to the standard Big Bang theory ie a light signal could not have had tjlne to pass from one point to another There s therefore no reason to expect them to have the same conditions The most plausible explanation advanced to explain this puzzle is called in ation In ation posits that at extremely early times maybe only 1038 s after the Big Bang an enormous injection of energy expanded the Universe exponentially The entire observable Universe derives from a tiny patch prein ation that was small enough to be in causal contact with itself Quantuln uctuations within that small patch seeded the formation of galaxies In ation predicts the relative strengths of different sized ripples in the CMB Those predictions are in excellent agreement with recent observations by the WMAP satellite Lecture 24 dark energy review appropriate parts of Chapter 22


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